Wind turbines with variable rotor speed are connected to the utility grid through a power converter that converts the alternating current produced in a generator (which as a general rule is coupled to the blades through a gearbox) to direct current with adjustable frequency and amplitude.
For high-power applications, it is common to implement converters comprising various modules interconnected in parallel, which allow greater controllability of power delivery in the grid. To achieve the highest possible power, the components in these converter modules are oversized; because such maximum power can only be delivered to the grid on very isolated occasions, optimization of the various modules is needed in order to ensure proper management of each operating point in the converter.
Specifically, in the case of variable-speed wind turbines in which the power produced by the generator depends directly on the wind speed, delivering high-quality electric power to the distribution grid is a complex process. In addition, the connection of the grid inverter to the electricity distribution grid generates harmonics which need to be eliminated, since otherwise they could cause a fault in the wind turbine with the resulting stoppage of the machine.
Different strategies are known in the State of the Art for enabling and disabling the converter modules of a power converter, which seek to resolve the appearance of harmonics in order to improve the efficiency and reliability of those converters.
One example of switching strategy for modular converters connected in parallel can be seen in US patent 2008/0031024, which describes a modular converter comprising at least two modules coupled in parallel and controlled by a means of control. The invention improves the influence of harmonics on the grid, which are caused by the converter's connection to the network, by connecting as many converter modules as necessary at a given time, i.e., based on the wind and the power to be obtained, it enables or disables the converter modules in order to deliver the required power to the grid.
Another control strategy is the one described by this applicant in European Patent EP1768223, which proposes a method for using a wind turbine converter system, including converter modules connected in parallel and capable of converting the electric power produced by a generator to electric power applicable to a utility grid. The method determines the enabling/disabling of the converter modules in response to parameters related to the variable amount of electric power being produced by the generator, such as, for example, temperature of the converter module's components, reference apparent power and reference active power for the converter system, a current reference to the converter system, a measured value of the apparent power produced by the generator or the apparent power delivered to the grid, a measured value of the active power produced by the generator or the active power delivered to the grid, and a measured value of the current produced by the generator or the current delivered to the grid.
Moreover, the State of the Art also envisages the possibility of disconnecting modules for a short or extended period of time and proposes the enabling/disabling of the converter modules by means of pulse-width modulation (PWM) patterns, in such a way that two or more converters are out of phase in relation to each other, thus reducing the harmonic components arriving from PWM and, therefore, improving the quality of the voltage signal applied to the utility grid.
However, this solution has one disadvantage: whenever it is necessary to vary the number of converters connected and, therefore, the switching phase difference set by the PWM patterns, such switching needs to be stopped, the different modules have to be reconfigured with the new phase difference, and they must then be reconnected. In other words, delivery of electric power to the grid needs to be stopped, since enabling or disabling a new converter module causes current peaks that set off the overcurrent alarms, and the wind turbine would enter emergency stop mode. These stops cause substantial production losses.